FMCW radars have been conventionally used as one of vehicle-mounted radars to detect any obstacle, a distance to a vehicle ahead, and a relative speed from the vehicle ahead.
The FMCW radar transmits a radar wave frequency-modulated so that its frequency linearly varies with time. It receives a reflected wave that is the radar wave reflected by a target, and mixes the transmission signal and the received signal to generate a beat signal. This beat signal is subjected to frequency analyses (FFT or the like) to detect the frequency of a peak component (beat frequency). The relative speed between the subject vehicle and the target that reflected the radar wave and the distance to the target are determined based on the beat frequency.
In case of FIG. 11, a vehicle-mounted radar Rs of a subject vehicle S receives not only the reflected wave resulting from the radar wave transmitted from itself (the radar R). It can also receive a radar wave transmitted from another vehicle-mounted radar Ri in any other interfering vehicle, such as an oncoming vehicle or a vehicle running aside (not shown). As a result, interference between vehicle-mounted radars Rs and Ri can occur. When such interference occurs, the following problem results: a beat frequency cannot be detected with accuracy, and a relative speed or distance is erroneously determined.
For solving this problem, JP-A-2002-168947 proposes an FMCW radar provided with an interference detection unit. This interference detection unit determines that interference due to some other radar has occurred in the following cases: cases where the amplitude of a received signal or a beat signal is higher than a preset threshold value for amplitude; and cases where a detected beat frequency (the frequency of the peak component of a beat signal) is higher than a preset threshold value for frequency.
Methods in which an amplitude is compared with a threshold value for amplitude are based on the following idea: when interference with some other radar occurs, a radar wave from the other radar is received in addition to the reflected wave associated with the relevant radar; therefore, the amplitude of a received signal or a beat signal is increased as compared with cases where there is no interference. Methods in which a frequency is compared with a threshold value for frequency are based on the following idea: when a frequency corresponding to the maximum value within a range of detection is set as a threshold value for frequency, it can be determined whether any abnormal input beyond the range of detection is present or not.
However, the methods in which occurrence of interference is determined from the amplitude of a received signal or a beat signal involve a problem of possible erroneous detection. In cases where the amplitude of an interference wave is low, a problem illustrated in FIG. 12A arises. That is, the amplitude of a received signal or a beat signal does not exceed a threshold value Vh for amplitude. Though interference with some other radar actually occurs, therefore, that interference is not detected. In cases where low-frequency noise (undulation) that is produced when a transmitting antenna and a receiving antenna are not sufficiently isolated or on like occasions is superposed, a problem illustrated in FIG. 12B arises. That is, the amplitude exceeds the threshold value Vh for amplitude. Though interference with some other radar does not actually occur, interference is detected.
Low-frequency noise could be removed through a filter. In this case, however, the following problem arises: a beat signal of a low frequency arising from a reflected wave from a target present at a short distance is also attenuated. As a result, the detecting capability for short range can be degraded.
The methods in which occurrence of interference is determined from a beat frequency (the frequency of the peak component of a beat signal) Fb involve the following problem: these methods are on the precondition that the subject vehicle and some other vehicle (the source of an interference wave RF) must be identical with each other in the modulation gradient of a radar wave transmitted by a vehicle-mounted radar (FMCW radar), as illustrated in FIG. 13A. In terms of practical use, occurrence of interference cannot be detected in almost all cases.
The reason is as follows: when there is a difference in modulation gradient, even if only slightly, between a received wave based on a transmitted wave from the subject vehicle and a received wave based on a transmitted wave from some other vehicle, a problem arises. The difference in gradient becomes the frequency of a beat signal, as illustrated in FIG. 13B. Therefore, the beat signal contains a wideband frequency component. As a result, when the noise floor of the beat signal rises, only a specific frequency component is not outstandingly enlarged for that reason alone. That is, it is not detected as frequency peak.
Also, the methods in which occurrence of interference is determined from a beat frequency involve the following problem, even when all the FMCW radars are identical in the modulation gradient of radar wave: in cases where some other vehicle's vehicle-mounted radar is not of FMCW, the phenomenon illustrated in FIG. 13C occurs. (Examples of methods other than FMCW include two-frequency CW, multifrequency CW, pulse, spread spectrum, and the like.) That is, a frequency component present in a beat signal under the influence of interference wave becomes undoubtedly wideband, and occurrence of interference cannot be detected.